fix: use configured base price in fulfillment total assertion

[XiaolongZhang-TT]

Description

test_fulfillment_flow in fulfillment_test.py hardcoded 3500 as the item base price when computing the expected total:

expected_total = 3500 + option_cost  # Base 3500 + shipping

Every other test in the suite resolves the item price via fixture_ctx.get_test_price(). The hardcoded 3500 only matches the default flower_shop fixture (valid_item.expected_price 35.00 → 3500); a fixture configured with a different item price would fail the total assertion even though the server behaved correctly — the same class of "default-only" bug as the recently-fixed hardcoded discount amount.

get_test_price() already returns minor units, so this is a pure substitution with no unit conversion.

expected_total = self.fixture_ctx.get_test_price() + option_cost  # base price + shipping

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, including removal of schema files or fields)
• Documentation update

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Is this a Breaking Change or Removal?

N/A — test-only fix, no schema/field removal.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (N/A — test-only fix)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules